Chlamydia trachomatis is a major sexually-transmitted human pathogen which produces a variety of important clinical diseases. The organism is an obligate intracellular prokaryotic parasite which undergoes a unique developmental cycle, alternating between two distinct morphologic forms: the infectious elementary body (EB) and the replicative reticulate body (RB). In this proposal we plan to investigate the molecular and genetic basis underlying this complex cycle, using the murine C. trachomatis strain MoPn. To do this we will (1) examine the temporal control of chlamydial polypeptide synthesis by 2D gel electrophoresis, to define families of constitutive and developmentally-regulated genes (2) clone representative constitutive genes, to allow examination of cis- acting signals minimally required for chlamydial gene expression, (3) clone representative developmentally-regulated genes, to define signals required for temporal regulation and to begin the identification of gene products which are involved in chlamydial differentiation. In parallel we will begin experiments to develop this organism as a genetic system. Specifically, this will involve the development of genetic vectors and transfection procedures to allow gene transfer into this organism.